Dot-matrix printheads have an array of printing elements that are selectively turned on and off by a printer controller to form printed shapes that are recognizable as characters. If one of the printing elements is inoperative, then the characters produced by the printhead will have a gap where the inoperative printing element would have produced a dot. If only one printing element is malfunctioning, the characters will appear deformed and unaesthetic. If several printing elements are malfunctioning, the print may be difficult to read. Unfortunately, printing elements can develop defects at any time without warning and the user must replace the malfunctioning printing elements immediately or accept inferior quality print.
Newer printheads for dot-matrix printers tend to have more printing elements so they can print at a faster rate with higher resolution. However, additional printing elements reduce the reliability of the system. If the probability of failure of each individual printing element is P and there are N printing elements, then the probability of zero nonfunctioning printing elements is (1-P).sup.N. Thus, the probability of all printing elements working properly decreases exponentially with the total number of printing elements. Hence, there is an increasing need for a fault-tolerant dot-matrix printing mode.
Some thermal ink jet printers, such as the Hewlett-Packard Desk Jet and Paint Jet printers, have a service station that restores printing elements having clogged nozzles. The service station primes the ink chambers of the printing elements and wipes the nozzle plate of the printhead to unclog the nozzles. If the service station clears a clogged nozzle, then that printing element can function normally. However, if the service station can not unclog the nozzle or if the nozzle is malfunctioning for some other reason, then the printing element will continue to malfunction and the printer will continue to produce inferior quality print. Most dot-matrix printers do not have any way of restoring malfunctioning printing elements.